1. Field of the Invention
This invention relates to a process for producing an aromatic dicarboxylic acid, such as terephthalic acid, isophthalic acid or phthalic acid of high purity by a liquid-phase oxidation of a benzene derivative, such as xylene, diisopropyl benzene, diethyl benzene or cymene, with a molecular oxygen containing gas in a lower aliphatic carboxylic acid as a solvent in the presence of an oxidation catalyst; and to an apparatus for use in this process.
2. Description of the Prior Art
An aromatic dicarboxylic acid, particularly terephthalic acid, has been used in great quantities in recent years in the manufacture of polyesters by direct polymerization with ethylene glycol. As direct polymerization techniques have advanced, the requirements as to the quality of terephthalic acid have become more severe, and terephthalic acid having high purity and a high degree of whiteness has been desired. Moreover, from the standpoint of resource and energy savings and pollution control, a process for continuously manufacturing high purity terephthalic acid in good yields over long periods of time has been desired.
For example, in the production of terephthalic acid by the known liquid-phase oxidation of p-xylene, a process for continuous production which comprises feeding p-xylene, a solvent, a catalyst and an oxygen-containing gas continuously into a reactor and meanwhile withdrawing the reaction product continuously from the bottom of the reactor is advantageous over a batchwise process because a product of constant properties can be obtained in a stable manner over long periods of time.
In the manufacture of terephthalic acid using a lower aliphatic carboxylic acid, such as acetic acid as a solvent, the terephthalic acid crystals gradually precipitate in the course of the reaction because terephthalic acid is only sparingly soluble in the aliphatic carboxylic acid solvent, and as a result, crystals adhere to the wall surfaces and other parts of the inside of the reactor. For this reason, it is inevitable in the conventional process that when reaction product builds up in the reactor, the operation should be stopped, and the inside of the reactor thoroughly washed with an aqueous solution of an alkali such as sodium hydroxide to remove the deposited material before the operation is resumed. When this method is used, stopping the operation naturally causes various disadvantages such as a decrease in the rate of operation, variations in the quality of the reaction product, and an increase in the cost of materials and utilities.
An economical and simple process and apparatus for performing the above reaction with good efficiency in which adherence of the aromatic dicarboxylic acid to the walls of the reactor is prevented is not at present known in the art.
When terephthalic acid is produced by feeding p-xylene into a reactor through a single material feed pipe, the product contains various impurities and has poor whiteness. Thus, unless the product is purified, the resulting terephthalic acid does not have a high purity suitable for direct polymerization.
Investigations have now been made into the cause of this difficulty, and it has now been found that when p-xylene is charged through a single feed pipe, the concentration of p-xylene increases locally. It has been confirmed that as a result, an oxidation reaction based on a radical chain mechanism takes place abruptly in areas where the p-xylene concentration is high, and an insufficiency of oxygen occurs in those areas, or a combination of radicals with each other takes place as a side reaction to impede the progress of the desired main reaction. Hence, the resulting terephthalic acid contains increased amounts of coloring products, or includes p-toluic acid and 4-carboxybenzaldehyde which are intermediates in successive reactions which can occur.
In order to obtain high purity terephthalic acid by reducing the amounts of the intermediate products in the above reaction, it is important to disperse the starting material sufficiently in the reactor at the time of charging the starting materials to the reactor, and to oxidize p-xylene to terephthalic acid smoothly and rapidly.
The same can be said with regard to other aromatic dicarboxylic acids as well.
It is known in general chemical reactions to feed a raw material into a reactor through a plurality of nozzles. With this method, however, it is impossible to feed the material uniformly from individual nozzles. In order to attain uniform feeding, a pump or a flow rate controller must be set up for each nozzle, and the apparatus and operation become complicated. Furthermore, the extent of dispersion through a plurality of nozzles is limited. These difficulties render the method extremely disadvantageous both technically and economically.
A method using a single nozzle with a number of pores is also known to improve the dispersion of a raw material without using a number of pumps or flow rate controllers. It has been found that this method produces the intended effect when dispersing the material in a gaseous phase, but that when the material is to be dispersed in a liquid phase as in the present invention, the feed material does not flow uniformly from the individual pores. Furthermore, it has been found that when the nozzle is used in a slurry, the pores become blocked.